Hello,
For the series resistor you must use the following value:
R= (Vbatt-Vleds) / Ileds.
Where:
Vbatt is the battery voltage
Vleds is the total forward voltage of the leds
Iled is the wanted current through the leds.
So in your case the resistor will be for a 10 mA current:
(4.5 - 4.2)volts...